The name of the debate, Measure For Measure, is a Shakespeare's play. It may also express one idea about the measurement in quantum mechanics or another, e.g. the correct idea that the measurement and the influence on the measured system is needed to find some information – to measure in the epistemic sense (to find out).

This conversation took place last night at the NYU Sourball Center for the Perforated Ass or something like that – I am no native speaker. All the talk about physics was introduced by the überhost, Brian Greene's wife Tracy Day, who introduced the host, Brian Greene, who began with tons of jokes about quantum mechanics in the mass culture and some nice, basic, a bit vague but effectively very accurate background about the double slit experiment which was enhanced by cool animations.

He actually does conclude that Born's interpretation of the state vector is right – which was a nice surprise for me. Brian's monologue was witty and accurate – just good. Well, up to 20:00 when he apparently completely switched to the usual "realist" bullšit.

Before that, he would admit that it's correct to say that the right interpretation of the wave function is just the probability (amplitude) wave. Suddenly, around 20:00, he says that the wave function doesn't agree with what we see, and that there are problems, and all this stuff. There are no problems and there are no disagreements, Brian. You just forgot what you were correctly saying a few minutes earlier – that the electron isn't really spread and the wave function isn't a classical wave.

At 20:45, Brian insists that a measurement "should" register both parts of a wave function – e.g. in the double slit experiment – at the same time. But it's just not true. If you understand that the wave function encodes the probability distributions, and you did just a few minutes ago, Brian, and if you know what the concept "probability" means, you should know that only one outcome among the mutually exclusive outcomes is actually realized. The other parts of the probability distributions – and other terms that used to be included in the wave function – are just not realized. They don't impact the events after the measurement at all. They were just potential outcomes but only one of them became the actual outcome and quantum mechanics – the correct, complete, and fundamentally most profound possible theory of Nature – predicts which of them may be the actual outcome by quantifying the probability for each potential outcome. What's so hard about it, Brian? Why all this gibberish about things not agreeing?

If you have a 1-in-14,000,000 probability to win a lottery and you are "really unlucky" and you don't win, the fact that the probability of a win was nonzero before the lottery was decided is forgotten. You just didn't win. Your financial planning should better be adjusted accordingly and immediately. If you didn't win, will you complain to the lottery company about the piece of the probability distribution that was allowing you to win but this piece of the probability distribution was neglected by the company after they picked the numbers? I hope you will not complain. It's the point of lottery – and life in general – that some potential outcomes just don't materialize, don't become actual outcomes, and their being possible a priori has absolutely no physical impact after that. In classical physics, the "not realized" outcomes described by a nonzero probability a priori were really impossible due to some precise arrangement of the degrees of freedom (the randomness isn't ever really random). In quantum mechanics, the explanation why a particular outcome wasn't realized cannot be deterministic and blamed on any objective properties of the system before the measurement (some of the randomness in quantum mechanics is fundamental and implied by the uncertainty principle) but it's true just like in classical physics that the not-realized outcomes must be forgotten even though their previous probability was nonzero!

It's a basic point of (Bayesian) probability that you adjust it once your actually learn some answers you didn't previously know.

The probability for your win is something that affects the informed predictions before the event is decided. But it doesn't affect any physics after the event is decided – simply because it was just a probability, a weight quantifying whether a potential outcome may become an actual outcome. If you don't understand why the not-realized parts of the probability wave don't and shouldn't impact any physics once it's decided that they're not realized, then you don't understand the concept of "probability", not even at the schoolkid's level, Brian!

Around 20:00, Brian switched from a proper description of quantum mechanics to Brian the Quantum Crackpot. Fortunately, this segment only lasted for 2 minutes or so. (Well, update: the word "fortunately" was way too optimistic because most of the stuff that 80% of the folks would say later was even worse.) He already invited the debaters – philosophers etc.

David Albert, Sheldon Goldstein, Sean Carroll, Rüdiger Schack.

Of course, one knows these men so it's not hard to expect what they would present.

Chapter one: quantum algorithm and its discontents

Wow, so quantum mechanics brings us "discontents". Is it a blasphemy to even consider the "possibility" that there is no "discontent" in quantum mechanics? It is not just a possibility. It's a scientific fact that competent physicists have been aware for nearly 90 years. Brian starts this "chapter" with some nostalgia – how the world got "screwed" when quantum mechanics came to the scene.

Carroll says that quantum mechanics was discovered remarkably quickly by the humans. Every part of it violated some "principle". Well, the pace at which quantum mechanics was discovered may look fast to some people today whose qualities simply can't compare to the qualities of the founding fathers of quantum mechanics. Quantum mechanics wasn't really too hard for those. Some of the key people – like Heisenberg, Dirac, and others – were less than 25 years old when they fully understood quantum mechanics in its present form. They not only understood it; they discovered the understanding. Some people considering themselves "experts" are still not even able to understand quantum mechanics even though many of them are 40, 50, or 75.

So Carroll's observations is good news but it is bad news, too. The bad news is that the intellectual quality of the physics community has dramatically deteriorated in the last 100 years.

The mathematician and QBist Schack is asked why Einstein disliked quantum mechanics and he seems to have no clue how to answer. A painful silence. Really painful. A complete loss of fluency. But he does say that Einstein thought that the states are real, and therefore the enforcement of entanglement apparently violated locality. This is promised to be discussed later.

Brian says that he got good grades in all QM courses but he is dissatisfied. He suggests he was just parroting something he didn't quite understand to be true in order to get good grades. Is quantum mechanics how it works? David Albert was asked this question and they didn't say anything clear.

"Why are students taught that (how to get predictions from QM) and not the actual issue?" Brian asked. But this is the actual truth, Brian. You really shouldn't have gotten good grades in your QM courses if you're not even open-minded enough to consider the idea that you should understand these things as they work.

Sheldon Goldstein, another philosopher who is sort of a Bohmian fan and whom I know from Rutgers, made a joke that instructors are teaching QM the way they do because they want the students to be as bewildered as the instructors were when they were young. Funny but not true. QM is being taught in the way it is – essentially compatible with the Copenhagen school's insights – because that's how Nature works and how science has to work for it to be operational. Some instructors fully realize this fact; others don't but they are being constrained by curricula that so far discourage a widespread proliferation of anti-Copenhagen crackpottery through colleges and especially grad schools.

Thankfully, the anti-quantum crackpottery has only taken over popular books, public discussions, and philosophy departments, not the actual scientific institutions. So far.

Around 31:00, Sheldon Goldstein said something that forced me to take a 2-minute break from this annoying show. Sheldon, why? He said that "he doesn't know what the probabilities should be probabilities of, so let's not talk about probability waves, let's just talk about waves". Holy cow. If you overlook that these are probability (amplitude) waves, you are missing the whole point. You are missing everything. You are wrong about the step 1. Goldstein clearly takes it for granted that the world follows classical physics and the wave is a classical wave that objectively exists. So then he asks why we only see one dot – why everything that his theory predicts seems to contradict observations. Well, the reason, Shelly, is that your theory is a pile of stinking šit. It is completely wrong. You won't be able to fix the mess that you want to see as a replacement for the correct, probabilistic theory.

According to Goldstein, Einstein was really troubled that the observer played a role. After all, it or she is made of the same stuff and it or she should obey the same laws. But in actual mechanics, she does. One may observer any other human being just like any other bound state of elementary particles that obeys the laws of quantum mechanics as strictly as any other bound state. An observer is also defining a "perspective" from which questions about observables are being asked and who may perceive the actual outcomes. But this fact isn't really adding any additional assumption at all. An observation is needed for something to be observed; if a "fact" isn't or can't be observed, it isn't really a fact and it can't be a player in the scientific explanations. Only things/facts that can be (at least in principle) observed may impact our beliefs about scientific questions. An observer is just a name for the broader set of degrees of freedom that is supposed to care about the results, or know about the results. He or she or it picks the questions. But no detailed properties of the observer are needed for the quantum questions (about the future observed values of observables) to be meaningful!

Brian shows some animations of the electron's wave function in Manhattan, something that I know from a book he wrote and I translated. The measurement of the electron "should" show a classical mixture of two possibilities, some overlapping juxtaposed images. But it shouldn't, Brian, because the wave function is no classical wave. It's a semi-finished product to calculate probability distributions. The word "probability" *means* that the potential outcomes that are not realized become exactly and irreversibly unphysical even though they carried a nonzero probability before the measurement.

Even though I was pretty good at playing the piano, even when I was 8 years old, I was convinced that by pressing the "C" key and the "E" key simultaneously, you must get something in between, like the sound "D". Fortunately, I was able to understand that "C+E" is something else than "D" a year later. I could hear it. In the same sense , the wave function combining the electron at the 5th avenue and the 7th avenue is something else than an electron at the 6th avenue. It's something else than half-of-the-electron at 5th avenue plus half-of-the-electron at the 7th avenue, too. Instead, it describes a single complete electron that is either on the 5th avenue or on the 7th avenue, with calculable probabilities. Not to be able to distinguish these three (or more) "ways to combine things" is an analogous mistake to my childhood's silly assumption that "C+E" must give us "D". ;-)

Press C-E-C-E quickly. It will still be different than D-D-D-D, right?

(With my childishly complete misunderstanding of the mixing of frequencies and the Fourier transform, you should understand how shocked I used to be by the idea – and by the observation – that my room contained all the waves that were needed to catch dozens of radio stations. It looked so surprising. But there are many different degrees of freedom and each frequency may remember its own intensity...)

Schack is the only person who is willing to say that this "problem" wasn't there and it was artificially fabricated later – he doesn't use these clear words. Too bad that he is not a native speaker and he is not excessively fluent, anyway. Schack is clearly the only one among the five men who realizes – along with Bohr and everyone else who has a clue – that one can't correctly formulate quantum mechanics as a science about objective, observer-independent statements; the information and predictions have to be formulated from the viewpoint of an observer.

Brian is throwing a dart. It becomes a virtual one on the screen and he hits the target.

Bohmian mechanics

Goldstein says that the de Broglie-Bohm theory conforms to what Brian was saying about the double slit experiment. Well, it agrees with most of the šit that Brian included in the description, plus some additional wrong things in between the lines that people like to add in their heads even if they are not being explicitly said. Goldstein seems to defend this pseudoscience by suggesting that most uninformed people in the audience would like it. Well, that's not a good reason to believe a theory in science, Sheldon. He sells the Bohmian picture and says that it ends up with the same predictions as proper quantum mechanics which is a complete lie.

The dart always has a clear position which is guided by the pilot wave. Except that it's easy to see that a particle can't have well-defined other quantum numbers, like the spin, because it would pick an objectively preferred \(z\)-axis in space and that would break the rotational symmetry. Also, in quantum field theory, the number of particles is variable – they may be pair-created and pair-annihilated – so it's clearly impossible that there exist specific classical positions of \(N\) particles. The number \(N\) isn't even well-defined. Moreover, two particles could never exactly hit each other and annihilate – the probability in classical physics for an exact hit is zero (which is still true even if there is some extra pilot wave affecting the classical particles' motion). Bohmists also fail to explain what happens with the "objectively real" pilot waves when the particle is measured or absorbed and how the initial state of the pilot wave is prepared. Their theory always inevitable contradicts the Lorentz invariance, prohibits one from choosing situation-dependent i.e. Hamiltonian-dependent bases that are relevant for different observations in different systems, and it just doesn't work at all. The Bohmian mechanics is just a sleight-of-hand meant to convince sloppy people that one doesn't need to abandon the pillars of classical physics – even though they have been clearly falsified.

Carroll offers some mildly critical comments about the Bohmian mechanics. Not really hitting the target.

Schack correctly points out that the Bohmian mechanics is almost certainly wrong because its basic classical object – the guiding wave – is in principle unobservable because a change of it should in principle impact things at a distance but it never does. So one really needs at least to fine-tune infinitely many things to make these a priori observable aspects of the pilot wave unobservable, to match the reality and avoid the contradictions with relativity, and even an infinite amount of fine-tuning isn't really enough to achieve this goal.

David Albert is "much more sympathetic" to the pilot wave theory than his two predecessors. He doesn't say anything about the particular issue, however. Instead, he correctly divides the pictures into the "realist" and "non-realist" ones. QBism is quoted as the non-realist one (even though the original quantum mechanics as formulated by the Copenhagen school was surely meant to be non-realist, too). The (wrong) realist pictures include the many worlds, objective collapse (spontaneous localization) theories (that Albert himself favors), the Bohmian mechanics etc.

A scoreboard with 4 rows (participants) rating the different interpretations. Carroll is neutral and Shack is thumb-down on Bohmian mechanics.

Many worlds interpretation

Carroll is supposed to defend this particular junk. His description is totally confusing – he is mixing many worlds with "shut up and calculate" and claims that the probabilistic interpretation emerges even if you don't start with it, and all these bizarre claims. There are no "many worlds" in what Carroll is saying in the first minute or two – but there is nothing meaningful in his presentation, either. Suddenly, he switches and he says that the other worlds (with outcomes of experiments not realized here) do exist. A one-minute break from this rubbish was needed again. Carroll adds comments that Everett left physics after Bohr et al. informed him that his musings were worthless. But according to Carroll, Everettians have "momentum" – well, because there are too many stupid people in the field these days.

He doesn't address the real problems of the many worlds at all – where the probability (weight) of the worlds is supposed to be encoded, when the splitting occurs, why, how can the objects re-interfere again if they were split, and so on. There are tons of ways to show that the fundamental reasoning involving "many objective real worlds" corresponding to potential outcomes of measurements is fundamentally wrong.

A lady rightfully asks how these different worlds differ from God and angels. Carroll says that God and angels don't have an equation. But that's just demagogy. One may pick the same equations – like Heisenberg equations – and say that this is what backs God and angels. It is exactly as illogical as everything that Carroll says about the many worlds interpretation backed by the proper equations of quantum mechanics.

David Albert mindlessly praises the Everettian memes. It's completely deterministic, it's so great. The only property that these people don't care about is whether these theories and adjectives are true. They only care about their feelings – whether the adjectives make them feel good – not about whether these adjectives allow the theory to agree with observations, at an accuracy that is competitive with the state-of-the-art description of various physical systems and disciplines. Albert says that probabilities don't exist, at least according to Carroll etc. – it's like asking an amoeba whether she's gonna "be" the individual on the left or the right after it splits. It will be both. That's funny because probabilities and their functions densities (cross sections, average energies in an ensemble, etc) are everything that may be predicted in quantum mechanics – in modern physics – and compared to observations. If these numbers don't matter, nothing is left out of science at all! Sean Carroll clearly doesn't care about science so he only says that this problem should be "focused on". He doesn't offer a glimpse of a possible loophole in this proof that the many worlds paradigm is totally wrong.

Quite generally, I am annoyed by this politically correct overlooking of falsification. Falsification should be the most essential step in the scientific method but the current reality looks very different. Someone offers a clear proof that an idea is rubbish. The defender of the rubbish theory says that "it's a problem that should be looked at" and people treat this "generosity" and "implicit promise of future positive answers" as being sufficient to allow the person to continue spreading the rubbished theory or even be paid for it. This is no science. In science, when a theory is falsified, it has to be abandoned, and no amount of would-be polite words should be able to circumvent this step.

These metaphors involving two amoebas make the situation look far less wrong than it is. They pick just two possible answers to a question that have large enough probabilities to be included as candidate answers. But it's a fact that almost every possible final state may evolve from an initial one with a nonzero probability. So the amoeba may also become a triplet of Elvis Presley clones on Mars or anything else. If you stop caring about the difference between high and low probabilities, the picture only says "absolutely everything goes". It doesn't say anything about Nature and our perceptions at all.

Schack's complaint about the many worlds is exactly the previous sentence I wrote – I wrote it before I heard Schack's response. He is clearly right about every question so far. Too bad that he has a deep rhetorical handicap relatively to his misguided fellow panelists.

Goldstein says that the many worlds approach has just one equation while the Bohmian mechanics has two and proper quantum mechanics has infinitely many (Carroll vehemently agrees with the latter claim). What a pile of rubbish. Proper quantum mechanics doesn't have any "censored" equations. Only the realists' stupid realist caricatures of it have hidden equations. The equations of quantum mechanics are what they are and they calculate the probability amplitude for every possible answer to every physically well-defined possible question and they're really the only things that may be scientifically meaningfully calculated and verified. Goldstein realizes that probabilities don't come out of the many worlds framework but he thinks that it's also based on a mistake. He thinks it's a mistake to think that the wave function is everything there is. Once you adopt Bohmian mechanics with the extra classical information (classical positions of the particle), you don't need many worlds. You also screw the physical theory in infinitely other ways but Goldstein doesn't care about it.

There can't be hidden variables that would decide about all the observed randomness in the world of quantum phenomena, Sheldon, because these hidden variables would have to act superluminally to preserve the correlations etc. which would contradict relativity. There are many other reasons why these extra hidden variables don't exist, too.

Goldstein says that a majority of physicists oppose the idea that the wave function is a probability wave. Depending on how you define "physicists", Sheldon. If you include philosophers (if I kindly avoid the word "crackpots") like you, a majority may oppose the fact that the wave function is a probability (amplitude) wave. But if you only include physicists who have at least a basic idea what they are talking about and what physics has been approximately doing in the last 90 years, all of them know that the wave function surely is a probability (amplitude) wave, not a wave conceptually analogous to the classical electromagnetic wave.

Carroll votes thumb-up on MWI, Goldstein is neutral, the other two are thumb-down.

Objective collapse

This particular delusion is being defended by David Albert who starts this portion with a confusing philosophical monologue about the boundary between the observer and the observed, or micro- and macro-. He doesn't say anything intelligent related to physics, as far as I could say. It's very clear that this Ghirardi-Rimini-Weber model predicts deviations from correct quantum mechanics that may be seen as unseen processes (frequent proton decay) or violations of energy conservation or something like that. Some of these pathological processes that falsify the GRW picture are guaranteed to occur at a significant rate if the parameters of the collapses are strong enough to prevent the wave functions from spreading so that the objects never spread to larger regions than where we apparently observe them (accuracy of measurements).

So it just doesn't work. And like all realist pictures, it violates relativity by fundamentally disallowing locality, and so on. The wave function is a classical wave in this picture, so the enforcement of entanglement – as well as the spontaneous localization event – does require a superluminal modification of an objective entity. That's not allowed in relativistic theories.

What I also find amazingly ironic is that these guys complain about the "unwanted boundary" between the (micro) observed systems obeying quantum mechanics and the (macro) observers who – these anti-quantum people think – don't obey them. And they offer a new picture. However, the truth is exactly the opposite. Proper quantum mechanics uses exactly the same laws for microscopic and macroscopic objects, whether they are human or conscious or not. It's not just a speculation. Most of modern condensed matter physics is about the (totally standard, "Copenhagen"-style) quantum treatment of macroscopic objects. It calculates the probability and the algorithm of the calculation is always the same. For different systems, the actual predictions will be different and there will be special effects for systems with many degrees of freedom, like decoherence, but the algorithm is completely universal.

On the contrary, GRW postulates a boundary they have to invent and add, some new effects that only affect large enough systems, and for those, one deviates from the simple equations of quantum mechanics. But no such deviations exist because the equations of quantum mechanics always work. Sometimes, classical physics becomes a relatively OK approximation but quantum mechanics remains valid and perfectly accurate in these situations! Moreover, these particular GRW-Albert-like cranks don't really have a clue and can't have a clue what the parameters describing the objective collapse should be. There is no consistent solution. For too frequent and dramatic "localization events", one violates the energy conservation and disintegrates protons etc. too often. For too infrequent ones, the wave functions keep on spreading to the usual, intuitively too large values of \(\Delta x\) so one still needs to interpret it probabilistically, anyway – because we just don't see these objects in a "fuzzy form". There is no solution because these objective collapses don't exist and the wave function is just not a real wave.

Again, Schack said the same thing as my previous sentence. All these realist pictures suffer from the same flaws; they can't depart from the flawed objective deterministic 19th century framework of physics.

Carroll says it may be true but he would retire if it were true because GRW adds lots of extra ugly baggage and because it is designed for mechanics and seems intrinsically incompatible with quantum field theory or string theory. (That's true for Bohmian mechanics and partly MWI, too.) Goldstein correctly points out that GRW is, unlike others, proudly saying that it gives different predictions than quantum mechanics. Whether these deviations may actually be falsified – the truth – is something that none of these guys seem to care about.

Carroll and Schack voted neutral on GRW, otherwise thumbs-up on "pursuing GRW". No one was negative. Holy cow. A near consensus that complete crackpottery should be studied in physics.

QBism

Schack advocates it by saying that probabilities are subjective, Bayesian, not objective. Brian says that he doesn't know where Schack is going. Really? It's totally obvious where Schack is going; the fact that one has to abandon the thinking about physics as describing an objective reality is the #1 fact one should understand if he wants to claim that he knows anything about the foundations of modern, quantum physics. Effectively, the objective thinking is often tolerable but those are approximations and the fundamental laws are ultimately about the intrinsic knowledge.

Even quantum mechanics preserves the basic notion of probability – I would write these things many times recently. They are degrees of personal belief about the future outcomes. Quantum mechanics just offers a new different engine to calculate them from the knowledge we collected in the past. Quantum states fully replace or generalize probability distributions. A quantum state or a density matrix is a compressed gadget that may be used to calculate the probability distribution for every quantity that may be measured – that's how the theory predicts – and nothing else is scientifically meaningful according to the new paradigm that's been settled for almost 90 years. Everything he says is totally right. It's just silly that he takes credit for these things that were the point of the original Copenhagen interpretation. And his rhetorical handicap is unfortunate, too.

Observations are just events after which we update our beliefs. Brian hired a guy at the Times Square to draw a caricature of Schack for $10.

Schack says that the goal of science is to predict experimental results. Because the Bohmian hidden variables or other worlds etc. are not needed to make predictions, a scientist just doesn't need to talk about them and doesn't really talk about them because they do nothing good to us. It is fully legitimate for a physical theory to declare these unobservable superstructures scientifically meaningless, and indeed, quantum mechanics does so and sort of needs to do so if we demand a high enough or complete accuracy or universality of the description. Brian asks how the Universe looked before there were any observers. Schack says that our cosmological theories are based on recent observations, anyway, and their goal is always to predict future observations. Even in cosmology. Schack implicitly says but doesn't emphasize that it's all about the subjective knowledge and the verification and prediction of patterns in this knowledge, not about drawing objective classical-physics-like pictures of the Universe in the past. It doesn't mean that we can't say anything about the Universe in the past. On the contrary. And of course that the expectation values of some quantities in the Universe (density of energy in radiation...) 400,000 years after the Big Bang have so small (quantum-related) errors that they may be treated as objective facts i.e. classical observables – because classical physics is a damn good approximation for certain questions. But there is no objective classical-like image that captures everything we may ever consider about the past state of the Universe. For a fine enough and accurate enough picture, we need the full description using quantum mechanics that postulates that the knowledge is inevitably and intrinsically agent-dependent.

Greene asks Schack where the spookiness goes in QBism. Schack correctly answers that the correlations in the EPR experiments must be in principle interpreted just as easily as the anticorrelation between the colors of Bertlmann's socks (a weird Viennese physicist who would never wear two socks of the same color). The entangled quantum state is simply a belief of the kind that guarantees the correlations between the two subsystems in various or all pairs of quantities we may look at. The actual cause of the correlation always depends on the common origin or mutual contact between the two subsystems in the past – for EPR pairs as well as socks – so there is no action at a distance that would be needed to "enforce" the correlation. Greene says that it's all about the observer and Schack corrects him that it's all about the relationship between the observer and the world. But yes, the perceptions are what the observer has. How else could it be?

Albert says that Bohr had proudly announced that the world was fundamentally less susceptible to the scientific penetration than before. Yes, no, what of it? Of course that it's true from some viewpoint. A theory in which one can only predict probabilities is less deterministic, and in this sense less susceptible to exact predictions, than a deterministic theory. (Even in Albert's favorite GRW theories, a random generator plays a role to ignite the spontaneous localization events, so the world in this picture would be less predictable than deterministic physics.) Many other things in quantum mechanics are actually much more accurate than they could ever be in classical physics. In classical physics, the hydrogen atom's energy would be fuzzy (not to mention that the atoms would collapse in less than a nanosecond). In quantum physics, the atomic energy has a sharp spectrum and the measurements bring us sharp answers. Atomic clocks achieve the relative accuracy of \(10^{-16}\). That would arguably be impossible to occur in a classical world because no system would naturally have "discrete eigenvalues" of an easily measurable observable. So quantum mechanics is fundamentally different than classical physics and the claim that quantum mechanics is always more fuzzy and less precise is already a bit of a demagogy.

Albert claims that Bohr's conclusion that the framework of physics had to be changed were "premature in many ways". The only problem with Albert's assertion is that it is a complete lie. There hasn't appeared a single piece of progress since the 1920s that would weaken Bohr's position and strengthen those that suffered from a classical physics nostalgia. Quantum mechanics as a framework continued to work perfectly well in all subdisciplines of physics while every particular enough alternative proposal has been falsified (along with huge classes like local realist theories, via Bell's theorem, large classes of non-local realist theories, and so on, and so on). The only thing that actually took place was the proliferation of deluded people such as Albert himself who are increasingly flooding the public discourse about quantum mechanics by junk and lies. Because they don't care about the agreement of their opinions with the observations at all, they increasingly spread wrong theories and commentaries at concentrations that are completely insane relatively to the degree of agreement of these theories with the empirical data – this degree may be described as "more or less total disagreement in all tests".

Schack reminded these folks that it's silly for the realist theorists to complain that QBism requires consciousness because their claim that they explain conciousness is "bold" – meaning bogus. This comment really means that one needs to assume that consciousness exists or arises if we want to explain all things including the existence of consciousness – and because we need to assume the existence of consciousness anyway, it's no problem for QBism to exploit this assumption. And QBism does describe the whole world – it's just an agent-dependent description, Schack says. And quantum mechanics is universally applicable – one observer may predict acts by another one and vice versa. Nothing is sacrificed and none of the extra structures from the realist theories are needed to explain anything and everything.

On QBism, Greene unexpectedly voted thumb-up. Albert says that the engine of QBism is vague but it had good spinoffs. Goldstein agrees with Albert. Goldstein insists that science has to be 19th century physics without any role played by the agents. Does he want to ban cars and cellphones, too? If he doesn't, why the double standards? In the 20th century, fundamentally probabilistic theories became at least as important for physics – and I would say more important – than cars became in the everyday life. Carroll says that he doesn't understand QBism, and indeed, he doesn't. He says that one needs a "mechanism". But one doesn't need any "mechanism", Mr Carroll. There is no "mechanism". It's really a point of QBism or a scientific, rational understanding of these questions that one is never obliged to add any of those "mechanisms" that wouldn't affect any testable predictions. Science is about making valid predictions of things that can be in principle observed (which may be observations of events that occurred in the distant past). It is not about inventing silly, Carrollian, childish pictures of "what is inside" that have nothing to do with things that the experiments actually address.

Will there be a consensus about these matters in 2100, Greene asks? Albert says that the serious discussion about these matters has only been taking place for 20 years. This is just plain bullšit. Einstein was wrong but the Einstein-Bohr debates were much more serious than the present ones and they took place much more than 20 years ago. The discussions about positivism that actually led the heroes to discover quantum mechanics were taking place about 90 years ago. They differed from this panel discussion in one dramatic way: stubborn zealots who would insist on preserving the 19th century foundations of physics whatever the evidence is didn't have much voting power in Munich, Göttingen, or Copenhagen. That's why some amazing physics could have been done over there. The value of everything that the likes of Albert, Goldstein, and Carroll – and even Schack who agrees with Bohr etc. – have ever done about the foundations of quantum mechanics doesn't reach 0.0001% of the value of the deep insights found by the founders of quantum mechanics. A little bit of modesty for these pseudoscientists would be helpful.

Albert guesses that there will be a consensus in 2100. Schack reproduces Mermin's opinions that everyone will accept QBism in 25 years so the field of quantum foundations will fade away. That's what I would want (perhaps faster) but I think that the human stupidity will continue to grow and the 2100 discussions, if there will be any, will be even more deluded and wrong than this 2014 debate. Goldstein predicts that QM will be completely replaced by something completely else (he means a new theory of 19th century-style classical physics) by 2100. Holy cow. Carroll believes that there will be a consensus. Applause.

Schack was the only person who knew what he was talking about but his rhetorical disadvantage was unfortunate. There are too many people who spend much more time by improving their rhetorical skills than by making their physics right and they are increasingly flooding the popular book market and similar panel discussions by delusions that are simply not right. They're completely wrong and unable to correct themselves because the agreement with tons of other people who don't care about the observations – group think – has become much more important than any scientific argument.

By the way, Sean Carroll and Charles Sebens posted a preprint that tries to pretend that they have solved something about deriving the Born rule in the many worlds interpretation. Needless to say, the paper is just a pile of incoherent junk. They want the different "parallel worlds" to be equally likely and not equally likely at the same moment, by their being marijuana-level fuzzy about the meaning of the probability and everything else. All these attempts are of course guaranteed to fail. The Born rule is very simple and may be shown to be equivalent to some other conditions of logical consistency. But one can't really derive it "out of nothing".

A function of the length of a projection of a state vector (onto the subspace where \(P=1\) for a projection operator corresponding to a Yes answer) is the only potentially good quantifier of the probability because that's what's conserved by the most general transformation – given by a unitary transformation on the Hilbert space which is by definition what preserves the lengths - and the total probability of all alternatives should be preserved at 100%. And the second power has to be chosen because of the Pythagorean theorem – two mutually exclusive answers connected by "OR" give a higher-dimensional subspace and the squared length obeys \(|c|^2=|a|^2+|b|^2\) by the Pythagorean theorem which is the only power that agrees with \(P_c=P_a+P_b\) appropriate for "C is A or B". And the mutual exclusiveness of two states is given by their orthogonality because orthogonality is what two eigenstates of a Hermitian operator corresponding to two other eigenvalues have to satisfy.

So the Born rule in quantum mechanics is logically inevitable (and it is not quite independent from other postulates of quantum mechanics) but the reasons are hiding in simple logic, probability calculus, and linear algebra in the previous paragraph, not 40+ page of unrestricted and internally inconsistent garbage by Carroll and Sebens that tries to mix these simple facts about probabilities with infinitely many parallel worlds and similar fantasies that have actually nothing whatever to do with the explanation of the Born rule and when analyzed carefully, inevitably contradict it. See also a similar criticism of the paper by Mitchell Porter. I am alarmed by the complete absence of any standards in that field. Two hacks may write 40 unregulated pages of completely worthless and inconsistent junk but be sure that it's enough for another hack to tell you tomorrow that these defects of the MWI approach have been addressed. Something about it is written somewhere so it's enough, isn't it? The fact that a competent physicist wouldn't use that preprint even as a toilet paper isn't important for those folks.

In the full green template, I added a new category of articles, "quantum foundations" (mobile), with something like 83 entries now. Please let me know if I forget to add this label at some moment. I've been trained by almost 10 years of practice to add certain limited labels only, decided not to add new ones. This is the first new label in many many years...

70 comments:

I have a minor suggestion for you. I checked the list of topics, it seems to be missing 'quantum mechanics'. You have written plenty of excellent articles on QM. I think the time has come to make it a topic on its own. This would make accessing older articles much easier.

What would be even better is if you could compile all your QM writings on Physics SE and this blog into one monograph. Many people would marry you out of sheer gratitude. I might :D

Thanks, a good point. It would be helpful if someone could help to cherry-pick them. It's easy to add the category. My problem is that people are not using the labels/categories much, anyway, so I don't want to spend too much time with that.

Notice that most of "quantum mechanics posts" also belong to the category of "philosophy of science", and this tag could be used for that matter, at least in the first instance. Aditionally, to search in a huge blog like this, Google is the key tool:

Hi Lubos. How you compare Consistent Histories and QBism. Is there an essential difference between them (didn't seem so to me) ?. By the way, I think Heisenberg has said the same thing Quantum Bayesians says, he even gave an analogy using airplanes.

Well, one reason is that you find researchers - at least philosophers - who will loudly claim that superluminal communications could be possible.

In this debate, David Albert defending the objective collapse theories as well as the Bohmian Sheldon Goldstein advocate theories that fundamentally do propagate information instantaneously so of course that they won't join your project to teach the media folks.

Schack arguably knows that current theories imply that no superluminal communication is possible, and so does hopefully Carroll in his vague version of many worlds although many worlds also have fundamentally nonlocal editions.

The empirical data clearly shows that relativity is right and superluminal communications are impossible. But if one demands an objective reality and the agreement with the tests of entanglement etc., he must believe that the information must be spreading superluminally. Because some people are incapable of accepting that the experimental data also show that the reality is not fundamentally objective, they are forced to believe that the superluminal transfer is in principle possible and at some level occurs all the time behind the scenes!

So it's not just the media folks who are deluded. It's a whole ring of people who are used by them as experts. It's almost always like that. Media never quite invent something by themselves. They always have some would-be experts who are the backers.

For me, all these correct interpretations - Heisenberg, QBism, Consistent Histories - are exactly the same theoretical framework. Consistent Histories are just optimized for asking a certain kind of multi-time composite questions but its proponents surely mean the same thing about the truth and predictions - they are predicted probabilistically and the collection of competing mutually exclusive histories is pretty much the same choice as the choice of an agent in QBism.

Thanks! I didn't know that those theories actually force superluminal communication to be possible.

Still, I think the public ought to be informed that many, many physicists say that it is not possible. Almost all people (laymen) I've met who express any interest at all (even very slight) in quantum developments think I'm a "science denier" or an "opponent of progress" if I criticize the idea of superluminal communication or travel.

Thanks for your answer Lubos. I think you should write a pedagogical article on arxiv, which shouldn't require any knowledge of quantum mechanics. What I am saying is that, you should start from the beginning and finish it. I don't believe it will be waste of time for you, because you are writing similar things over and over. You shouldn't write pages whenever someone says wrong things about foundations of quantum mechanics (excluding nobel prize winners :) ), life is too short for that. I don't think there exist a complete text for foundations for quantum mechanics which also properly derives classical mechanics and accessible to beginners. I am sure that you are capable of writing such a text and I believe it is a win-win situation for everyone.

The problem in our times (due to the Internet and the abundance of information at every level) is that the general public fails to distinguish between established and widely accepted state of the art physics from pure crack-pottery. Under the general phrase "physicists say" you can find all sort of crazy ideas and speculations. This is truly harmful since it confuses people and does not allow them to understand the way the physical world is structured/functioning and the importance of new ground breaking discoveries for our deeper understanding of this physical world.

In other words it is hard to recognize a diamond when it is hidden deep inside a pile of crap.

Lubos, your unending responses to this QM interpretation craze display an amazing fortitude, I’m starting to fear for your sanity. To me, you effectively sum it all up with [The equations of quantum mechanics are what they are and they calculate the probability amplitude for every possible answer to every physically well-defined possible question and they're really the only things that may be scientifically meaningfully calculated and verified.]

Something the QM interpreters might ponder is this: Why is it that you never hear of anyone interpreting a mathematical theory, say the meaning of Galois Theory? I think the answer is that it is logically correct and that is all there is to it. I think physics is the same but additionally includes one very critical element: given a logically correct theory, does it make quantitative predictions matching empirical reality. If so, great, move in that direction, if not, move in another. All this interpretation stuff is basically meaningless: only our quantitive predictions allow contact with reality.

If you can play Flight of the Bumble Bee on extreme right keyboard, you can play it on extreme left keyboard - except for the sound. (frequency)(wavelength) = k. Low frequency plus short duration must be muddy.

Perhaps. Quite likely, in fact, but the presumed higher average intellectual quality of physicists 100 years ago is not the reason why quantum mechanics was "discovered quickly". The reason is that they had a lot of data to constrain their imaginations. The standard model of particle physics also got discovered fairly quickly, and there was a lot of data in this case too.

Incidentally, there also exists a way to reply to your complaint by saying that it is wrong. One of the ways to describe some of the new capital hiding in quantum mechanics is to say that probability densities may be negative for a while, see

What's odd is that basic quantum mechanics is not that hard. In fact it is amazingly simple linear mathematics. Even solving Schrodinger's equation is not that hard under certain assumptions. So I don't think it's IQ that is the problem. It is more an attitude towards quantum mechanics. It's a sort of desire for some quasi-religious mysticism. Maybe with the decline of traditional religion some people are trying to create a new scientific religion by hyping up science to be the new mysticism and quantum mechanics is the weirdest part and so the part that is most easily mystified. Lots of people love the whole Dancing Wu-Li Masters bull shit. I did when I was 17 but have grown up now.

I once asked Lubos to compile his QM work into a book but for some reason he does not seem interested. Someone needs to pull apart all of these crazy QM interpretations once and for all. A blog is not enough.I am sure it would be a big hit.

Thanks. That helps a lot. I think I'm starting to slightly understand how to truly look at the world. But how should one think of a measuring device—as any bound group of particles (e.g. a human, an orangutan, etc.) complex enough to make a measurement that gives the device a subjective perception (and that, if the device has language, the device can say 'exists')?

That is interesting. If you excuse a naïve question: What happens if you measure (?) Wigner distribution around a particular point where it is negative? Will it be true that within the uncertainty limit it will always be positive?

Much too boring to watch. I lasted about five minutes. "Wenn ich Kultur höre ... entsichere ich meinen Browning!"...(Hannes Johst)---substitute the words "the meaning of QM" or "the interpretation of QM" for "Kultur". I agree with Gene now that I have studied QM more and read Dirac's book (well, most of it anyway). Rather than get into this discussion, which nearly always leads to flaming, I think folks should stick with QM axioms and ensuing mathematics rather than make philosophical statements. I do find it amazing though that actual physicists seem to think there is an actual collapse of some wave that is not a probability amplitude. It seems clear to me.

a measuring apparatus is a bound state of elementary particles obeying the laws of quantum mechanics just like any other object. When you ask a question about the apparatus, you answer it using the very same algorithm as if you ask a question about the hydrogen atom - in principle.

If you want to find out whether a measuring apparatus affects another system, once again, you are using the same laws - applied to the whole union of the apparatus and whatever else should be influenced or whatever else is a part of the physical evolution.

The measuring apparatus differs by its extra ability to "see" some physical observable, and this extra ability may be derived from the same laws of quantum mechanics, too. For example, one may derive why the sensitive pixels of a digital camera do what is expected.

You may finally come to the question of consciousness. Every observable may be thought of as being able to be aware of itself. However, in most cases, one may see that the physical system won't be able to remember or take the observable into account for some smarter decisions, so this "rudimentary consciousness" is useless.

What's more important is that the other objects' consciousness has absolutely no impact on you. You know that you are aware of the state of some of your brain cells, to say the least, and quantum mechanics agrees with that and allows you to organize what you are actually aware of.

At the end, I believe that the only science-level meaningful questions about consciousness are those about neuroscience - how the brain etc. works, what it is capable of, where the signals are going, how they react - and these may be in principle reduced to the same laws of quantum mechanics. One may still calculate the probabilities whether a neuron will switch to one voltage state or another if the eye sees something etc. - still just another quantum mechanical task involving bound states of atoms.

One may feel that there arguably exists a more spiritual level of consciousness "above" neuroscience but science has nothing to say about it now - and it is true both for proper "subjective probability"-based quantum mechanics as well as the wrong "realist" pictures. The latter don't explain the spiritual consciousness, either.

But what I read in between the lines is the idea that the spiritual consciousness must be assigned by a "stamp" or a "criterion" to objects, and it's this spiritual consciousness that decides on whether things becomes real or "collapse". But that's not true at all. Whether some interference pattern is destroyed or not - a sign of measurement by a photon - is determined by the standard probabilistic laws of quantum mechanics, not by any metalaws or added laws. All these things are calculable. The chance to regain an interference pattern is lowered due to decoherence, and all these things are calculations within totally standard quantum mechanics without any added structures. The "purely spiritual" part of consciousness of other people or objects doesn't affect your perception at all! From this viewpoint, you may very well adopt sollipsism without worsening any aspect of your ability to predict.

probabilities and probability distributions are not "observables" in the technical sense. They may only be measured by making many measurements of the same physical system in the same initial state. What you actually measure in these individual measuremenets isn't rho(x,p) but those "x,p" themselves, and you may reconstruct the probability distribution from the frequencies of x,p.

Now, x.p cannot be measured simultaneously, which is why Wigner's rho(x,p) isn't a real distribution but just a quasidistribution. You may at most design an approximate measurement that will measure both x,p with some errors delta x, delta p that obey the Heisenberg relation, delta x * delta p is greater than hbar/2. When you do so, when this unavoidable uncertainty is taken into account, the Wigner distribution is "smeared" and you the smeared distribution will predict non-negative counts for every possible outcome of the experiment.

You should be named Pope of the Church of the Copenhagen School of Quantum Mechanics by now. Always enjoy the clarity of your articles on QM, and your ongoing crusade to convert the lost souls or burn the heretics.

Out of topic, but what do you think about Stephen Hawking predictions for the World Cup?

Dear Lubos you right that it is meaningless to talk about spiritual level of consciousness because it is out of our understanding limit but stating that consciousness emerge from brain is contradicting with whole point of quantum mechanic that there is not a objective reality out there prior to observation ,you can not say whole world is subjective but your body and brain is objectively real .way i understand quantum mechanic is that we are dreaming this world and whole universe including our body and brain is a perception of our consciousness . this is very easy to imagine i think everyone have a experience of dream in

I've never said that your brain or any other brain is "objectively real".

In the exact quantum description, the knowledge of all variables describing your brain or any other brain is as subjective as any other knowledge of any other variables.

But it's this knowledge about the brain - your brain, any other brain, or any other apparatus - that allows one to probabilistically predict how the brain or anything else reacts to any initial conditions etc.

One way a wave function seems more real to me as a chemist is how geometrically well defined molecular orbitals exist that involve so much rapid electron motion within their 95% density volumes that effectively they do seem to be valid as proper physical objects, despite merely being claimed to be probability functions, measurement be damned.

And the journalists as well as the audience of too popular media channels can also not discern between real by the international physics community acknowledged physicists, and pompous overreaching fools down to trolls, who enjoy the limelight and are capable of speaking in an arrogant authoritative tone even though (or because?) they blatantlyy lack any deeper knowledge...

Dont ask have many times I have seen even the Trollking cited as a theoretical physicist, etc ... :-(

I just ran into a rather new BBC documentary, Quantum physics and microscopic Universe

https://www.youtube.com/watch?v=JADhIuKxAwE

which is literally *flooded* with the sentence that in the microscopic world, information travels faster than light. The TV folks didn't make this up. They clearly had to be okayed to say - and repeat - this utter misconception by tons of the deluded physicists who can be seen on the show, too.

This delusion is so wrong but it is so widespread - not only among the pure laymen - that one can't really eliminate it anytime soon.

Well, while slightly off-topic, we must recognize that it is a possible inner cause of why so many educated people and with a high level of scientific training are falling into such serious confusions. Usually they share other things besides militant atheism: a clear progressive liberal tilt, an acceptance of anthropogenic global warming at some degree, etc ... sometimes correlation does imply causation... ;-)

Yep, fine, that's the downside of being generic... but there are many with those three common factors, Leonard Susskind and Victor Stenger come immediately to my mind, but I'm sure there are many more.

What motion do you think there is for an electron in a one-particle self-consistent field construction like an orbital? An orbital is a tiny part of the full N-particle electronic Hamiltonian eigenstate of the molecule, which is supposed to be an stationary state. You can't get more abstract, information-made-tool, non-observable, than an orbital.

Lubos - Thanks for the link. I've watched it with captions, not audio. (BTW, in one place, the automatic captioning renders some word or phrase as "masturbated.")

Based on captions only (they seem pretty complete), I can't agree that it's "flooded" with statements that information travels FTL. I'm far more impressed with what I see as a general difficulty of talking about quantum theory; it seems almost unavoidable to speak as if the world were classical, so much so that the speakers may have tricked themselves into error over the course of their careers.A person can't keep speaking as if something were the case without running an enormous risk of coming to think that it is the case, or double-thinking a la 1984.

The probe tip is only being pushed by the electron clouds that are described as being atomic orbitals and their delocalized combination into molecular orbitals. The nuclei are hidden inside as little dots much smaller than these big blobs.

http://www.rsc.org/images/AFM_350_tcm18-161978.jpg

The correspondence of the computer-calculated molecular orbital probability objects and the real physical images is exact. If it quacks like an object....

Pseudo science oftentimes seems to trump real science with journalists. The criterion for publishing an article is primarily how sensational or controversial it is, not how well the study was done, the study's actual conclusions, how the study ranks versus the other evidence already peer reviewed and published, and the credentials of the research team members. This is pure speculation, no new data was used in the study, or created by the study itself, no empirical evidence is cited to justify the conclusion reached, and no explanation of why the other studies might have been flawed were presented. The public very much needs and is entitled to demand, a corp of qualified, objective, scientist reporters. More often than not, what we see and what we get is someone who happened to take a mid-level science course in college posing as a science reporter.

Apologies, I don't understand why you think that being "objective" has anything to do with the self-interference.

The interference occurs for the same *mathematical* reason why classical waves interfere. However, the interpretation of the wave function is completely different than the interpretation of a classical wave. It is a wave of probability amplitudes which are ultimately used to produce all subjective probability distributions by an observer who previously learned something about the electron that told him what the state vector was.